More and more offshore wind farms are being built in European waters and significant numbers are planned elsewhere around the world. In general, the offshore wind farms built to date are relatively near to shore or in shallow waters; however the water depths that they are being installed in are: increasing. While, the majority of turbines currently installed offshore have power outputs typically in the range from 3-5 MW, they have consistently been increasing in size and a number of larger turbines are now being developed. Due to the trend for larger turbines in deeper water depths, the technical capabilities of the turbine foundations used to date are proving limited. As such foundations are becoming larger and heavier in order to be able to withstand the greater forces experienced.
The predominant foundation type in the industry to date is known as a monopile. It consists of a large diameter steel tubular section which is piled into the seabed. The process of piling is both lengthy and noisy, and induces significant stress to the structure reducing its fatigue life. In addition, environmental concerns about the noise emitted during installation can restrict the opportunity to conduct piling both seasonally (due to the impact on the environmental receptors, such as marine mammals and fish, including their spawn) and over the day/night cycle (due to the impact on human receptors).
As both turbine sizes and water depths increase, the cost of monopile foundations and the technical difficulties associated with their installation also increase. While larger diameter monopole solutions have been proposed to accommodate larger turbines and deeper waters, the forces required to install them increase significantly, as such so do the noise and induced stresses. In addition the forces experienced by the structure through the action of waves, tides and currents also increase significantly. As the hydrodynamic forces increase so do affects such as the scour of the sediments surrounding the structure. All of these issues add to the design requirements for the foundation adding cost in terms of the materials required to withstand the stresses at the same time as limiting the stresses imparted to the seabed.
In addition to the above, the types of seabed that are suitable for monopiles are limited. Certain geological types, such as chalk, do not interact well with the stresses and moments imparted to the seabed. Chalk, for example, crumbles when stressed (as during piling, or when loaded and unloaded by a cyclical moment) and thus does not exert the desired forces upon the pile. Similarly other seabed types do not have the desired structural capacity to support the pile, or the pile's design is required to be modified to reduce the forces imparted to a suitable level, increasing cost.
While substructures and foundation types other than monopiles have been used, these have so far been used in limited numbers. The use of substructures such as gravity bases and jackets has been limited by water depths, wave climates and the cost associated with their manufacture. To date other structures such as tripods and tripiles have only been used in very limited numbers.
Gravity bases have predominantly been used in the Baltic Sea where the water is relatively shallow and the wave regime is not as energetic as in the North Sea. They are large, typically concrete, structures often filled with aggregate to add mass. The restricted use of gravity bases is because of the cost of their manufacture, which is linked to the materials, space and time required to produce them. Large construction areas are also required, for longer periods per foundation due to: the nature of casting a concrete structure-of significant height. In addition, the quantity of material involved, especially when designed for deep or energetic waters, adds to the cost and manufacturing time and increases the spatial requirement of proposed sites.
As gravity bases are such massive structures, the issues associated with them are similarly significant. Once installed the size of the submerged structure leads to significant hydrodynamic effects, such as scour of the seabed and the forces experienced by the structure. As the hydrodynamic forces are so significant, the design of the structure is required to be large enough to accommodate them, adding mass and cost. In addition, as the structure is submerged, any mass that is added has associated buoyancy which means that it is less effective in exerting a force to act against those imposed upon the structure, thus the full capacity of the mass inherent to the structure is not utilised and the overall size of the structure needs to be increased to offset the effect of buoyancy, in turn increasing the forces experienced by the structure. As the forces experienced by the structure are significant so are those imparted to the seabed as such, prior to installation gravity bases typically require seabed preparation to ensure a level surface and reinforce the load bearing properties of the sediments. Such seabed preparation increases cost, time and risk of the offshore operations.
Jacket foundations have been used in deeper waters, although their cost has limited the number of installations. Jackets are steel lattice structures that are secured to the seabed through the use of piles. Their cost is associated with the space required to manufacture them, the amount of time and effort taken to weld the structure, the amount of material required as well as the cost of piling and installing the jacket. As significant tensile and compressive forces are transferred to the lattice structure of the jacket, the detailed engineering and fabrication of the top section of the jacket is a significant aspect contributing to the cost of the overall structure. The requirement to secure die jacket to the seabed through the use of piles means that the noise implications of installation remain a significant concern for the industry.
A few other substructure types have been used, or proposed, for offshore wind turbines including, tripods, tripiles, and various floating designs. Tripods used, or proposed to date, have or would, require significant amounts of fabrication work as-well as piling, both of which drive up costs. Tripiles also require piling, and the detailed engineering to distribute the stresses at the top of the structure is complex and costly. Floating foundation designs have been prototyped however they require significant water depths, or entail significant fabrication and material costs.
While the oil and gas industry only require a few installations per field, the relatively high cost of the structures themselves can be absorbed into the overall cost of the business model. In the case of offshore wind, due to the high number of structures required, it is in part these high costs that currently prevent the industry from becoming a mature and competitive technology. It would be advantageous to make offshore wind competitive by haying a structure that is low cost, simply and speedily manufactured with lower spatial requirements than current foundation types, quickly assembled, and may be easily deployed to a location of choice. It would be further advantageous if the structure could be installed with little or no noise emission during the installation process, and if limited or no seabed preparation was required. It would be highly advantageous if that structure were also relatively stiff and capable of handling a range of forces exerted upon it, to enable an assortment of turbine types to be installed thereon.